The present invention relates generally to medical devices and methods. More specifically, the present invention relates to ultrasound catheter devices and methods for treating occlusive intravascular lesions.
Catheters employing various types of ultrasound transmitting members have been successfully used to ablate or otherwise disrupt obstructions in blood vessels. Specifically, ablation of atherosclerotic plaque or thromboembolic obstructions from peripheral blood vessels such as the femoral arteries has been particularly successful. To disrupt occlusions of small blood vessels, such as the coronary arteries, ultrasound catheters must typically be sufficiently small and flexible to permit their advancement through the tortuous vasculature of the aortic arch, coronary tree, or other similarly narrow vasculature. Thus, safely and effectively disrupting or ablating obstructions from coronary arteries with ultrasound energy devices depends largely on the diameter and flexibility of the ultrasound catheter employed.
Various ultrasonic catheter devices have been developed for use in ablating or otherwise removing obstructive material from blood vessels. For example, U.S. Pat. Nos. 5,267,954 and 5,380,274, issued to the inventor of the present invention and hereby incorporated by reference, describe ultrasound catheter devices for removing occlusions. Other examples of ultrasonic ablation devices for removing obstructions from blood vessels include those described in U.S. Pat. No. 3,433,226 (Boyd), U.S. Pat. No. 3,823,717 (Pohlman, et al.), U.S. Pat. No. 4,808,153 (Parisi), U.S. Pat. No. 4,936,281 (Stasz), U.S. Pat. No. 3,565,062 (Kuris), U.S. Pat. No. 4,924,863 (Sterzer), U.S. Pat. No. 4,870,953 (Don Michael, et al), and U.S. Pat. No. 4,920,954 (Alliger, et al.), as well as other patent publications W087-05739 (Cooper), W089-06515 (Bernstein, et al.), W090-0130 (Sonic Needle Corp.), EP, EP316789 (Don Michael, et al.), DE3,821,836 (Schubert) and DE2438648 (Pohlman). While many ultrasound catheters have been developed, however, improvements are still being pursued.
Typically, an ultrasound catheter transmits energy from an ultrasound transducer through a transducer horn and then a transmission member, such as a wire, to a distal head. Ultrasound energy propagates through the transmission member as a sinusoidal wave to cause the distal head to vibrate. Such vibrational energy is typically utilized to ablate or otherwise disrupt vascular obstructions. To effectively reach various sites for treatment of intravascular occlusions, such ultrasound catheters often have lengths of about 150 cm or longer.
One difficulty related to transmission of ultrasound energy through long catheters is premature wear and tear and breakage of the catheter body, the ultrasound transmission member, or both. In general, an ultrasound transmission member or wire must be flexible enough to be passed through various areas of the cardiovascular circulation, but must also have sufficient strength to transmit energy to the catheter tip to ablate vascular obstructions. A stronger, more durable transmission wire allows for greater transmission of energy and is more durable than a thinner wire, but it may not be flexible or thin enough to be advanced through the vasculature to a desired treatment area. A thinner wire is less durable and more susceptible to breakage.
Currently available ultrasonic transmission wires typically break toward the distal end of the ultrasound wire, where the cross-sectional area of the wire becomes smaller. Wire breakage is generally caused by stress concentration due to transverse vibrations and fatigue. When ultrasonic energy is conveyed through the transmission member to the distal head, the head vibrates in both a longitudinal direction (back and forth in the direction of the longitudinal axis of the catheter) and a transverse direction (back and forth perpendicular to the longitudinal axis of the catheter). The longitudinal vibrations typically create the beneficial effects of disrupting an occlusion, while the transverse vibrations are predominantly unwanted artifact that stresses and fatigues the transmission member. One goal in developing ultrasound catheters, therefore, is to dampen transverse vibration of the transmission member while still providing an optimal level of longitudinal motion.
One proposed solution for limiting transverse vibration to prevent wire breakage in ultrasound catheters is to place one or more transverse vibration absorbers near the distal end of the wire or around the smallest cross-sectional area of the catheter, near its distal end. Such a solution is significantly limited, however, by the structural requirements of an ultrasound catheter. Typically, an ultrasound catheter is a small, single lumen tube, which requires continuous irrigation to cool the wire while ultrasound energy is delivered. Placing one or more vibration absorbers at or near the distal end typically increases the diameter of the catheter, interferes with the continuous irrigation system, or both.
Several prior patents describe such transverse vibration absorbers. For example, U.S. Pat. No. 5,397,293 (Alliger et al.), hereby incorporated by reference, and U.S. Pat. Nos. 5,380,274 and 5,267,954, previously incorporated herein by reference, describe catheter devices having a distal head affixed to the catheter body. Affixing the head to the catheter body acts to limit transverse motion of the head. With such an affixed distal head, however, more ultrasound energy is required to produce a desired amount of longitudinal vibration to disrupt or ablate a vascular occlusion. Ironically, increasing the ultrasound energy applied to the transmission wire may actually cause increased stress on the wire and, consequently, premature wire fatigue and breakage.
Another challenge in developing ultrasound catheters is to provide sufficient mechanical energy at the distal head to break through calcified plaque. Intravascular plaque is often composed of calcified material so hard that treatment devices typically cannot pass through them. At the present time, neither ultrasound catheters nor any other comparable devices have solved the problem of calcific plaque occlusions in blood vessels.
Therefore, a need exists for ultrasound catheter devices and methods that allow for ablation or disruption of vascular occlusions, including hardened calcifications. Ideally, such catheter devices would be sufficiently thin and flexible to be advanced through narrow, tortuous vasculature, such as the coronary vasculature, while also being configured to enhance the usable life of the ultrasound transmission wire within the catheter. Such devices would preferably provide sufficient longitudinal vibration of a distal catheter head for disrupting calcific plaque and other occlusions, while minimizing stress to the ultrasound transmission member or wire caused by transverse vibration.